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In Slimer, Is That You?, during a freak accident, Egon's mind and Slimer's mind are exchanged hours before Egon is due to enter a test of mental power with the ghostly Master of Shadows. Can Egon reverse the process, even as his personality deteriorates into a quest for food?[2]

The Ghostbusters were out on a daytime bust. Peter and Ray looked around for Egon. Suddenly, a nearby building's window shattered. They exchanged looks and ran for the entrance. Egon was indeed inside, hiding behind crates while analyzing his P.K.E. Meter's readings. Ray, Peter, and Winston gathered by Egon, who is oddly calm. Egon revealed he was picking up secondary and tertiary readings just like the last few cases. He deduced an outside force was controlling and directing the ghosts. A voice congratulated Egon. The Ghostbusters peered over at the ghosts. They ceased all movement and vanished. A large entity materialized and introduced himself as the Master of Shadows.

The guys opened fire on the Master but he teleported away. The Master is insulted by the move and declared he planned on destroying New York City. After the Master disappeared, Egon studied his P.K.E. Meter intently. Back at the Firehouse, Egon completed a rush job on a new device. Lacking time to properly insulate the wires, Egon went ahead and transmitted on the Master's spectral frequency to open a channel of communication and learn more about the new enemy. After getting his answering machine, the Ghostbusters were able to make contact with the Master. He applauded Egon's ingenuity and revealed attacking the city was only a way to demonstrate his superiority. The Master added it wasn't personal.

Egon decided to egg the Master into challenging him. Once the Master took the bait, Egon accepted and proposed a midnight showdown at the corner of 15th and Madison. The Master agreed and stated if Egon didn't show up, he'd destroy the city in one night. Slimer prodded the holographic projector. His ectoplasm shorted out the circuits, fused them, and caused Slimer and Egon's minds to be exchanged. In the kitchen, the guys tried to figure out how to reverse the process in time. However, another call came in. Egon stayed behind to lay out some calculations while Janine kept an eye on Slimer.

The three Ghostbusters fought several ghosts at a roller derby. They moved to fast to be captured by the streams so Winston elected to run over to the other side. However, a pair of skates accosted Winston and forced him to join the derby. Peter used his years of watching derbies and advised Winston to get ahead of the ghosts. Once he did, Peter told him to shoot the skates. Winston tumbled forward but had a clear shot at the ghosts. Peter and Ray jumped onto tracks and trapped them. The Master of Shadows appeared and inquired where Egon was. Meanwhile, Egon finished his calculations but struggled in Slimer's body. He even slimed Slimer by mistake.

Janine and the other Ghostbusters arrived in Egon's lab to check on his progress. Egon stated he needed Ray to rewire the necessary wires. In the meantime, Egon accompanied Peter and Winston on the next call. However, while on on the bust, Egon paused at a candy shop's display and briefly reverted to Slimer's ravenous personality. To make matters worse, Egon couldn't adjust to the Proton Pack's recoil. Back at the Firehouse, the Master appeared to Slimer and Ray. The Master moved up the appointment and demanded Egon's presence immediately. As Ray raced downstairs to update the others, Slimer ran out of the Firehouse to meet with the Master.

Ecto-1 returned to pick up Ray and the rewired device. They took off but Egon ate an old candy bar he found in the seat. At the wheel, Peter declared it was best if they hurried. As Slimer waited at 15th and Madison, the Master of Shadows appeared. Unaware Slimer was in Egon's body, the Master was perplexed by Slimer's gibberish. However, he clearly understood the raspberry Slimer blew him. The Master threw blasts of P.K.E. at Slimer. Ecto-1 pulled up and Egon flew out to distract the Master while Slimer was hauled off to safety. Slimer is hooked up and Ray activated the device. It worked and both Egon and Slimer were returned to "normal." Still utterly confused, the Master was easily confined and trapped. Back at the Firehouse, Egon and Slimer played a prank on the guys and made them think the transfer didn't entirely work. Once the coast was clear, both enjoyed some pizza.

This episode was re-dubbed with Dave Coulier and Kath Soucie as it was meant for the syndicated, but was switched with the ABC year 2 shows. The original surfaced in the UK versions of the show with Lorenzo Music and Laura Summer as the voices of Peter and Janine. Later in America it reaired on USA and Fox Family with Music and Summer. The re-dubbed version with Dave and Kath was not included on the DVD. Multimedia samples can be found on Spook Central's Alternate Episodes page and as of 2017, can be viewed in full on Netflix in the United States.

Egon tells Peter they must first know the enemy. This is a tenant from the "Art of War" by Sun Tzu, "It is said that if you know your enemies and know yourself, you will not be imperiled in a hundred battles..."

After Egon awakens in Slimer's body, he recites the third law of Newton's Laws of Motion, "For every action there is an equal and opposite reaction."[6]

↑Peter Venkman (2009). The Real Ghostbusters - "Slimer, Is That You?" (1987) (DVD ts. 10:12-10:17). Time Life Entertainment. Peter says: "Looks like all that roller derby I watched as a kid is going to payoff."